1. Field of the Invention
The invention is in the field of fluid flow meters and in particular ultrasonic flow meters.
2. Description of the Prior Art
In the velocity measurement of fluids flowing through a pipe line it is desirable to provide an arrangement for detecting the fluid flow, usually liquid flow, in a non-invasive fashion and with considerable accuracy and with mechanisms which are relatively inexpensive and long lived. It is also desirable to have a measuring device which will easily afford a plain readout unaffected within wide tolerances by variations in temperature and comparable ambient conditions. Ultrasonic flow meters have been known in the past, as for example those described in the U.S. Pat. No. 3,738,169 to Courty and U.S. Pat. No. 3,935,735 in the name of Bock W. Lee assigned to the same assignee as the instant invention. As explained more fully in the aforesaid patent, it is conventional to utilize a pair of simultaneously transmitted ultrasonic burst signals directed along opposite directions in a path between axially spaced receivers. The received burst signals are processed to obtain a time signal, t, representative of the time between the simultaneous transmission and the first received signal, and a transit time difference signal, .DELTA.t, representative of the difference in time between the pair of received burst signals. The desired velocity flow measurement is then computed utilizing a .DELTA.t divided by t.sup.2 calculation which provides an indication of fluid flow independent of temperature and comparable ambient conditions.
In systems of the type described above, it is desirable to maintain an unambiguous phase relationship between the pair of received burst signals so that corresponding cycles of the received burst signals may be compared to obtain the transit time difference signal .DELTA.t. The requirement of a unique phase determination upon the received burst signal imposes a predetermined upper limit on the transmission frequency f utilized in making up the ultrasonic burst signals. However, additional consideration must be given to the selection of the ultrasonic frequency in view of the continual presence of gas bubbles in the moving liquid.
It is known that the presence of gas bubbles in a liquid can introduce signal attenuations as high as 60 db or more per centimeter for frequencies below one megahertz, while transmissions with relatively low attenuations can be achieved reliably for frequencies above a few megahertz. This phenomenon is generally explained by considering the thermal relaxation of the gas bubbles and the surface tension forces effective during the collision of one bubble with another. Due to the compressibility of the entrapped gas, a gas bubble is set into vibration by the ultrasound. Depending on the size of the gas bubble, the vibrations may be close to or in tune with its natural thermal relaxation frequency, which is inversely proportional to the diameter of the bubble. To the extent that these vibrations represent energy absorbed from the ultrasound wave, maximum signal attenuation occurs when the ultrasound frequency is resonant with the relaxation frequency of the gas bubble. In the aggregate, maximum signal attenuation occurs when the ultrasound frequency coincides with the characteristic relaxation frequency of the gas bubbles corresponding to the peak of the bubble size distribution.
Consideration of surface tension forces effective upon the collision of one gas bubble with another leads to the conclusion that the gas bubbles tend to merge into a larger bubble rather than breaking up. Thus, regardless of the initial distribution of bubble sizes, the distribution in time will move toward the larger sizes, and the absorption spectrum of the aggregate will move toward the lower frequencies below one megahertz.
For many practical applications, the entrainment of gas bubbles in the flowing liquid is unavoidable. In order to insure transmission with reasonable attenuation and achieve signal to noise ratios adequate for precise measurement, the ultrasound frequency should be chosen above a few megahertz. However, the desirability of having an unambiguous phase determination for comparing received pairs of signals imposes a selection of frequency less than a calculable predetermined amount. The upper limit established by the predetermined amount (for example a 100KHz limit) often conflicts with the requirement of high frequency transmission for low attenuation, especially in the region of large pipe diameters and/or high fluid velocities. Thus, on the one hand, the phase ambiguity condition requires the frequency be limited below a certain upper limit in order to make the extraction of transit time information from the accumulated phase shift unique. On the other hand, the absorption spectrum of entrained gas bubbles requires the selection of a frequency above the megahertz range in order to insure ultrasonic transmission with reasonable attenuation and thereby achieve adequate signal to noise ratios required for precise measurements. The instant apparatus and method solves the conflicting problems in the selection of frequency and permits both adequate signal transmission strength and unique phase determination.